Warmbier was arrested in January last year after being caught on video taking down a government poster and keeping it as a souvenir.

The North Koreans arrested him and charged him with committing a hostile act. His appeals for mercy were ignored.

For his parents, it was heartbreaking to watch from afar as he stood in court begging for mercy and already showing serious signs of psychological problems.

Speaking through tears he told the court, "I made the worst mistake of my life".

But there was no forgiveness and he was sentenced to 15 years hard labor.

When he arrived back in the US, Warmbier's father walked onto the plane and couldn't believe what he found.

"We looked in and Otto was on the stretcher,” he said.

“He was jerking violently making these inhuman sounds. Otto had a shaved head. He had a feeding tube coming out of his nose.”

It was clear from the brain damage that he'd been a victim of psychological torture or had been administered various damaging drugs.

Fred Warmbier described how his son was staring blankly into space and jerking violently.

“He was blind and deaf and it looked like someone had taken a pair of plyers and rearranged his bottom teeth."

He died three days after coming off the plane.

President Donald Trump watched the interview and afterwards said, “Otto was tortured beyond belief by North Korea”

Mr Trump also repeated his threat to shut down the North Korean nuclear program by military force if necessary.

North Korea has denied mistreating Warmbier.

Three Americans accused of various crimes against the state are behind bars in North Korea, which is engaged in a tense standoff with the Trump administration over its banned missile and nuclear weapons programs.